Phase change couplings are known for engine camshafts that allow the phase of intake and exhaust camshafts to be changed relative to one another or relative to the crankshaft in dependence upon the operating conditions of the engine. All such couplings require power to change the camshaft phase and this is derived directly or indirectly from the engine. In particular, if the phase change coupling is fluid pressure operated, the engine is required to run normally in order to provide the necessary fluid pressure.
Most of the camshaft phase change couplings currently in use have no means of locking the camshaft in a known angular position when there is insufficient actuation pressure to control the position of the phase change coupling.
The camshaft phasing couplings incorporating locking mechanisms that are known generally take the form of a locking pin that engages in a slot or hole when the phase change coupling is in its "start-up" position. In the case of an uncontrolled engine shut down, the phase change coupling may not be able to return to the start-up position and so the lock will not operate. With these existing types of phase change coupling, no amount of engine cranking will allow the phase change coupling to move to the correct position if there is insufficient control pressure because the cranking will always tend to retard the camshaft timing.
If the engine attempts to start while the phase change coupling is incorrectly positioned, poor emissions may result, or in the worst case the engine may not start at all. On account of recent emissions legislation, the risk of high pollutant levels upon engine start-up may not be tolerated.